The present invention relates to an image forming apparatus for executing an electrophotographic copying process. More particularly, the present invention relates to an image forming apparatus capable of preserving the wear resistance of a photoconductive element or image carrier thereof, image reproducibility and image quality despite a repeated charging process and a repeated developing process.
A problem with a photoconductive element included in an image forming apparatus is that the chargeability of the element is lowered due to repeated operation and, in turn, deteriorates image characteristics. The deterioration of image characteristics include background contamination particular to a reversal development system. Specifically, when toner contained in a developer is charged to polarity opposite to expected polarity, it deposits on the unexposed portion of the photoconductive element (white area in the case of a positive image) and thereby contaminates the background of the element. Further, the toner deposits even on the defective charged portions of the white area during development, appearing as fine black dots in the resulting image. This is particularly true with a digital image forming system that forms a latent image on the photoconductive element in the form of dots by, e.g., selectively turning on a beam spot or turning it off in accordance with an image signal.
Background contamination described above is ascribable to the deterioration of the chargeability of the photoconductive element, which is ascribable to the repeated operation of the element, as known in the art. Specifically, when a charging system using a scorotron charger or similar corona discharger, charge roller or similar charging means charges the photoconductive element, it generates ozone, nitrogen oxides (NOx) and other produces due to discharge and deteriorates the photoconductive layer of the element. Moreover, the thickness of the photoconductive layer decreases due to mechanical hazards occurring in the apparatus.
There is an increasing demand for a photoconductive element having a thin photoconductive layer for enhancing image quality in an electrophotographic process. A thin photoconductive element prevents a latent image from spreading therein and thereby enhances the reproducibility of thin lines and fine dots. A thin photoconductive layer, however, lowers the chargeability of the photoconductive element, limiting a margin with respect to background contamination.
To cope with the decrease in the chargeability of the photoconductive element while reducing the thickness of the photoconductive layer, there has ben proposed a method that adds additives having various antioxidant effects to the outermost layer of the element, which includes a charge holding layer. This kind of method is taught in, e.g., Japanese Patent Publication Nos. 50-33857 and 51-34736 and Japanese Patent Laid-Open Publication Nos. 56-130759, 57-122444, 62-105151, and 3-278061.
Japanese Patent Laid-Open Publication No. 6-003921, for example, proposes a system that directly injects a charge in the photoconductive element in order to protect the photoconductive layer from e.g., ozone. Specifically, the system applies a voltage to a magnet brush or similar conductive member and causes the conductive member to inject a charge in a charge injection layer in contact therewith.
With the charge injection type of system described above, it is possible to effect substantially 1:1 charging with respect to the voltage applied to the conductive member. The system therefore reduces ozone and NOx more than conventional contact charging systems other than the charge injection type of system. Moreover, the system reduces the deterioration of the photoconductive layer and therefore reduces background contamination even when the photoconductive layer is thinned.
The charge injection type of system, however, has the following problems left unsolved. The photoconductive element includes a charge injection layer formed by dispersing tin oxide or similar metal oxide in resin. Therefore, irregular dispersion of the metal oxide, for example, causes the surface of the photoconductive element to be irregularly charged. Further, a charging member, a developing member and an image transferring member contact the photoconductive layer. The resulting stresses acting on the photoconductive layer deteriorate it and limit the durability of the photoconductive element. Moreover, when the charging member is implemented by a magnet brush, it charges the photoconductive element only in the region where magnetic particles forming the magnet brush contact the element. It follows that to uniformly charge the photoconductive element, it is necessary to increase the number of points where the magnetic particles contact the surface of the element.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-230652, 7-168385, 7-239565, 8-89149, 9-211978, 9-329938, 11-72934, and 11-149204.
It is therefore an object of the present invention to provide an image forming apparatus producing a minimum of ozone and NOx and capable of charging a photoconductive element with a minimum of power.
It is another object of the present invention to provide an image forming apparatus free from background contamination despite the use of a thin photoconductive layer and stably operable over a long period of time.
It is a further object of the present invention to provide an image forming apparatus capable of enhancing the durability of a surface protection layer formed on an image carrier and including a charge injection layer, and uniformly charging the image carrier.
An image forming apparatus of the present invention includes a photoconductive element including a conductive support rotatably supported and a charge injection layer and a surface protection layer sequentially laminated on the conductive support. A charger includes a conductive body for injecting, when a preselected voltage is applied thereto, a charge in the charge injection layer in contact with the surface protection layer. A writing unit exposes the charged surface of the photoconductive element imagewise to thereby locally vary the potential deposited on the photoconductive element and electrostatically form a latent image. A developing unit develops the latent image to thereby produce a corresponding toner image. The toner image is transferred from the photoconductive element to a recording medium. Assuming that the charge injection layer has a thickness of D micrometers, and that the potential deposited on the surface of the photoconductive element by the conductive member is V volts in absolute value, then a ratio V/D is confined in a preselected, range that does not contaminate the background of the photoconductive element.